1. Field of the Invention
The invention relates generally to the art of reducing sulfur dioxide with a metal in the presence of water to form a metal hydrosulfite salt solution. In particular, the invention relates to the use of iron metal to reduce sulfur dioxide in aqueous medium to produce an iron hydrosulfite liquor and also to the conversion of the iron hydrosulfite liquor to a sodium hydrosulfite bleach liquor.
2. Prior Art
Hydrosulfite (dithionite) salts are widely used commercially as reducing and bleaching agents. Clay, textile and pulp manufacturers are principal users. Considerable prior art is directed to the production of hydrosulfite salts by reducing sulfur dioxide or bisulfites with a metal. Of the metals that have been suggested, powdered zinc is the only one that has enjoyed widespread commercial use. Zinc hydrosulfite liquors have been produced commercially for many years but they now have limited utility because zinc is expensive and, furthermore, it is now recognized that zinc is a water stream pollutant which is toxic to marine life. Processes for converting zinc hydrosulfite liquors to sodium hydrosulfite are well known but the process is costly. Sodium hydrosulfite liquors can be produced without intermediate formation of zinc hydrosulfite by using sodium in the form of a mercury amalgam as the reductant but this method requires an inexpensive source of sodium amalgam such as a chloro-alkali plant would provide. Anhydrous solid sodium hydrosulfite is produced commercially by reaction of sulfur dioxide with formates in organic solvents.
It has long been known that iron can be reacted with sulfur dioxide in the presence of water to produce a hydrosulfite salt. In fact, it is understood that iron hydrosulfite was the first hydrosulfite salt that was ever prepared. Iron is included in lists of metals capable of reducing sulfurous acid in several early patents; for example, U.S. Pat. No. 791,675 to Descamps (1905). The metals disclosed in this patent include "zinc, iron, tin and the like." British Pat. No. 181,132 to Stubbs (1922) mentions platinum, iron and zinc. U.S. Pat. No. 1,472,828 to Eustis (1923) discloses "zinc or other metallic dust." Platinum, iron and zinc are mentioned in the disclosure of prior art in U.S. Pat. No. 2,149,506 to Bump (1939). In all of these patents, zinc in the form of a dust is preferred. Iron is mentioned only fleetingly and the patents fail to disclose or suggest reaction conditions that could be used with iron as the reductant. The only detailed report dealing with the preparation of iron hydrosulfite of which I am aware is in a publication of Murray et al. "THE BRIGHTENING OF GROUNDWOOD PULP WITH CALCIUM DITHIONITE," PULP AND PAPER MAG. CAN. 68, T-28, January 1967. Recognizing the abundance of the lost cost of iron as well as the limited nature of the information in the literature concerning the use of iron to form hydrosulfites, the authors reported that iron hydrosulfite could be produced from iron powder and sulfur dioxide at low temperature but the yields reported were far from quantitative.
It has also been suggested in the prior art to employ bases to precipitate metal hydroxides from metal hydrosulfite liquors and produce other hydrosulfites. For example, Descamps suggested calcium, sodium and barium bases for use with metal hydrosulfites, especially zinc. Eustis disclosed caustic soda. U.S. Pat. No. 2,172,275 to Dehnert (1939) reacted aqueous solutions of heavy metal hyposulfites (sic) of which he included salts of zinc, cadmium and iron, preferably zinc, with an alkali metal amalgam to produce a solution of sodium hyposulfite (probably hydrosulfite). Murray et al. describe in limited detail the conversion of their iron dithionite liquor to a calcium dithionite liquor by reaction with calcium hydroxide followed by filtration to remove the precipitate of ferrous hydroxide and by-product calcium sulfite. Murry et al. proposed the use of sodium hydroxide to precipitate the iron but no experimental work is described in their publication.
To the best of my knowledge iron has never been used on a commercial basis to produce hydrosulfite liquors in competition with zinc hydrosulfite bleach liquors or dry sodium hydrosulfites produced by various technologies. The prior art as set forth by Murry et al. reveals many problems which would preclude use of iron to produce hydrosulfite liquors of commercial utility. At the outset, very fine iron powders were needed to produce iron hydrosulfite in yields greater than 70%. For example, the best yield reported (75%) was achieved with 450-500 mesh iron powders. Using iron powder as fine as 250 mesh, yield was only about 65%. Such powders, especially the 500 mesh powder, are quite costly. Furthermore, very concentrated sulfur dioxide solutions (in excess of 15%) were necessary to achieve yields in excess of 70% even when using the 500 mesh iron powder. Large excesses of sulfur dioxide were apparently used (as evidenced by the relatively constant low pH values during reduction). Since the reaction had to be carried out at temperatures below 5.degree. C. for high yields, problems such as achieving effective cooling of the solution during reduction would be encountered. Finally, the prior art technique for removing soluble iron from the iron hydrosulfite liquors with lime led to significant losses in hydrosulfite yield. For example, when iron hydrosulfite was prepared with a 75% yield, use of lime to precipitate iron led to recovered yields of calcium dithionite liquor of only 50% at best. While Murray et al. suggested that sodium hydroxide could be used as a precipitant, I found that significant losses in yield of hydrosulfite liquor are generally encountered when using sodium hydroxide to convert iron hydrosulfite to the sodium salt.
Accordingly, an object of my invention is to provide a process for producing iron hydrosulfite liquor in high yields (for example, 70 to 80% based on the iron used) in which relatively coarse, inexpensive commercial iron powders can be used. Another objective is to provide a technology for converting such iron hydrosulfite liquors into sodium hydrosulfite liquors useful for bleaching. In particular, an object is to produce sodium hydrosulfite at high recovered yields, for example about 70% recovery, for a liquor in which essentially all of the iron has been removed.